It is known to use, as the light-sensitive substance in photoprinting materials, benzene diazonium compounds carrying in 4-position to the diazo group a tertiary amino group which is substituted by alkyl or aralkyl groups, or whose nitrogen atom is a component of a heterocyclic radical.
These compounds may contain further substituents in the benzene ring, e.g. alkyl or alkoxy groups, preferably in 2- or 5-position to the diazo group.
The properties which render these compounds suitable for diazotype purposes are largely determined by these substituents.
Thus, the light-sensitivity of the compound is considerably increased by the presence of an alkoxy group in 5-position to the diazo group, while the same group, when in 2-position, decreases the light-sensitivity of the compound.
A combination of one alkoxy group each in the 2- and in the 5-position of the benzene ring, which results in highly light-sensitive and at the same time stable diazonium compounds, has proved especially suitable for practical purposes. The light-sensitivity may also be influenced by the type of the tertiary amino group in the 4-position. It is increased, for example, by the presence of a pyrrolidino radical in this position. The radicals attached to the nitrogen atom affect mainly the coupling speed of the diazo compounds. Lower alkyl groups cause a lower coupling speed than do those with longer carbon chains or aralkyl groups. Heterocyclic radicals instead of a tertiary amino group with aliphatic radicals also normally cause an increase in the coupling speed, but there are wide differences in this respect, e.g., between the readily coupling morpholino compounds and the relatively slow-coupling pyrrolidino compounds. As a further possibility of increasing the coupling speed, the carbon chains of the alkoxy groups in the benzene nucleus may be extended. The readily coupling compounds are used mainly for one-component materials, while the less rapidly coupling compounds are used for two-component materials.
Recently, it has been found that the coupling speed, particularly of highly light-sensitive photoprinting materials, is not sufficient for certain purposes. Thus, for processing two-component materials, photoprinting apparatus is available which is equipped with a powerful light-source which renders it possible to use relatively short exposure times. When development, e.g. ammonia development, is to be effected in the same apparatus, the working speed utilized in exposure cannot be maintained in the development step, because in this case the disproportionately low coupling speed requires a longer resident time of the material. Nor is it possible to balance this drawback simply by using a more active coupling component, because this would mean an undue restriction of the choice of color shades available. This drawback is particularly evident in the case of materials with transparent supports, e.g. plastics films or lacquered transparent papers, because the necessary quantities of water vapor and ammonia penetrate less easily into the surface of these materials than into that of normal photoprinting papers.
Also in the case of one-component diazotype materials, there is a tendency to use diazo compounds of greater coupling speed. Here it is, above all, the desire to replace the alkaline developer solutions normally used hitherto by solutions having a neutral or weakly acid reaction, because such solutions are considerably more stable than alkaline solutions. However, the coupling reaction proceeds considerably more slowly at lower pH values, so that the choice of highly active diazo compounds available for this purpose is very limited.
It is known from Belgian Pat. No. 676,820 that diazonium compounds having a trifluoromethyl group in 3-position to the diazo group exhibit a higher coupling speed than do unsubstituted compounds.
It has also been suggested in U.S. Pat. No. 3,539,347 to use diazonium compounds derived from unilaterally diazotized p-phenylene diamine and containing fluorinated alkoxy radicals in o-position and/or in m-position to the diazonium group, which radicals contain no further substituents besides fluorine and hydrogen atoms.